1. Field of the Invention
The present invention relates to a differential amplifier circuit comprised of a differential pair of transistors and a driver circuit with the square-law characteristic for driving the pair and more particularly, to a differential amplifier circuit to be formed on semiconductor integrated circuits, which has a transconductance linearity within a wide input voltage range.
2. Description of the Prior Art
A differential amplifier circuit having a superior transconductance linearity within a comparatively wide input voltage range is known as an "Operational Transconductance Amplifier (OTA)".
A first example of the conventional OTAs was disclosed by Schmook in the IEEE Journal of Solid-State Circuits, Vol. SC-10, No. 6, PP. 407-411, December 1975, in which there is a disadvantage that its transconductance becomes low in value. The example contains two unbalanced differential pairs of bipolar transistors in which a relative emitter area ratio of the transistors of each pair are four (4), and input ends of the transistors of each pair are cross-coupled. The linearization of the example is known as the "Multitanh" technique, and the number of the transistor pairs has been increased to enlarge its input voltage range.
A second example of the conventional OTAs was disclosed by A. Nedungadi and T. R. Viswanathan in IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS, Vol. CAS-31, No. 10, pp. 891-894, October 1984. The example is of a Complementary Metal Oxide Semiconductor (CMOS) structure and contains a first differential pair of MOS transistors and a squarer for driving the first differential pair. The first differential pair is driven by the output current of the squarer to compensate its nonlinear term in its output characteristic. The squarer includes second and third unbalanced differential pairs of MOS transistors in which a relative ratio of a gate-width W to a gate-length L ratio (W/L) of the transistors of each pair are 2.155, and input ends of the transistors of each pair are cross-coupled and output ends thereof are parallel-coupled.
In the paper, they made mistakes in circuit analysis, and in their analysis model the relative ratios of a gate-width W to a gate-length L ratio (W/L) of the transistors of the second and third pairs were made equal in value to each other and constant current sources for driving the respective differential pairs were also made equal in current value to each other, so that the analysis was extremely difficult to understand. In addition, since it is very difficult that the relative ratio of (W/L) of each pair of the squarer is made to be 2.155 exactly, nobody has utilized the example of Nedungadi and Viswanathani yet.
For example, to realize the relative ratio of (W/L) of two MOS transistors 2.155 exactly in LSIs, one of the transistor is required to be formed of 200 in number of unit transistors connected in parallel and the other thereof is required to be formed of 431 in number of the unit transistors connected in parallel.
A third example of the conventional CMOS OTAs was disclosed by the inventor Katsuji Kimura in IEICE TRANSACTIONS ON FUNDAMENTALS, Vol. E75-A, No. 12, PP. 1774-1776, December 1992. The example was made based on the fact that a quadritail circuit or a quadritail cell comprised of two pairs of MOS transistors and one constant current source for driving the two pairs could be employed as a squarer. The MOS transistor pairs are driven by an output current of the quadritail cell to compensate its nonlinear term in its output characteristic.
In the example, there is an advantage that a current mirror ratio is only required to be made at most 2 and this example of the OTA is easy to be integrated. Additionally, there is another advantage that the example is easy enough in circuit analysis for any LSI designers to understand.
A fourth example of the conventional differential amplifiers was developed for realizing a gyrator filter by Krummenachet and Joehl in 1988. In the example, there is a differential pair of first and second MOS transistors and a third MOS transistor is provided between the sources of the first and second transistors. The transconductance of the differential pair can be improved in linearity due to the fact that the electric resistance of the third MOS transistor varies in accordance with its input voltage.
A fifth example of the conventional OTAs was disclosed by Wang and Guggenbuhl in IEEE Journal of Solid-State Circuits, Vol. 25, No. 1, PP. 315-317, February 1990, in which a quadritail cell is used.
The OTA is an essential functional element in analog signal processing and its superior transconductance linearity is very important. However, with the above-identified examples of the conventional OTAs, it is difficult to ensure superior transconductance linearity within a wide input voltage range without enlarging its circuit scale.